A quartz glass optical fiber is generally manufactured by forming an optical fiber base material called a preform and drawing the preform. In the manufacturing of a preform, a fine glass particle deposited body formed of an aggregate of fine glass particles is dehydrated and sintered by heating. As methods for manufacturing the glass particle deposited body, a vapor-phase axial deposition (VAD) method, an outside vapor deposition (OVD) method, and the like are known, for example.
In the OVD method, two or more burners are arranged around a long core material along a longitudinal direction of the core material, and a raw gas containing SiCl4 and so on is supplied into oxyhydrogen flames generated by the burners. Thus, fine glass particles are generated and deposited on the core material to form a fine glass particle deposited body. Thereafter, the fine glass particle deposited body is dehydrated and sintered in a furnace to obtain a preform.
Regarding the aforementioned method for manufacturing a fine glass particle deposited body, Japanese Patent Application Laid-Open No. Hei 3-279234 discloses that gas conditions of burners arranged at equal intervals across the overall length of a glass rod are individually controlled such that the shape of the deposit can be equalized. Such control of the gas conditions of the respective burners uses mass flow controllers (MFCs) provided for the respective burners.
Moreover, Japanese Patent Application Laid-Open No. Hei 8-284904 and Japanese Patent Specification No. 5090646 describe the use of a flow divider and the like to branch a gas flow into a plurality of partial gas flows, in order to ensure that the gas is fed to the burners with high uniformity and reproducibility.
As in the case of the method for manufacturing a fine glass particle deposited body for optical fiber, a plurality of supply targets such as burners arranged along a longitudinal direction of an object such as a core material manufacture a product while being supplied with a gas such as a raw gas, a combustible gas, and a combustion-supporting gas, in some cases. In such a case, in order to ensure uniformity and stability of the manufactured product in the longitudinal direction, it is required to supply the gas with highest possible uniformity to the plurality of supply targets.
However, in the method using the MFCs provided for the respective supply targets as described in Japanese Patent Application Laid-Open No. Hei 3-279234, it is difficult to eliminate influences of aging changes in MFCs and of an individual difference among the MFCs.
Moreover, in the method using the flow divider as described in Japanese Patent Application Laid-Open No. Hei 8-284904 or Japanese Patent Specification No. 5090646, influences of processing accuracy of the flow divider itself are not negligible. Also, it is difficult to eliminate a resistance difference among pipes caused by an individual difference among the pipes. For this reason, even with the method using the flow divider, it is difficult to perfectly equally branch the gas.